The present invention relates to a container for holding high purity isopropyl alcohol (IPA) used in the electronic industry such as the production of semiconductors and the production of liquid crystals which extremely dislike the inclusion of metal impurities. More specifically, the present invention relates to a container for holding high purity IPA even after the high purity isopropyl alcohol is stored for a long period of time, the total amount of metal dissolved can be suppressed below 10 pg/ml (ppt) based on the high purity IPA.
The quality of a container for holding IPA used in the electronic industry is generally a glass or plastics in the case of containers of small capacity, whereas in the case of medium to large containers having a capacity of at least 10 liters, stainless steel is generally used by considering strength-safety.
In the electronic industry, especially in the semiconductor industry, with an abrupt technical innovation of a miniaturization of the form pattern size and the increase of the diameter of a substrate, high purities are required in IPA and other chemicals used for washing and drying, and the amounts of these chemicals to be handled are increasing.
When a large quantity of IPA is handled, a metallic container such as stainless steel is ordinarily used as a container for holding IPA. However, when IPA has a high purity, the contamination of IPA by a metal dissolved from the metallic container poses a problem in a state of being stored in the holding container.
Especially, in the production of semiconductors in recent years, with the miniaturization of the device, conditions against the contents of metallic impurities (including a mist or a particle) in IPA used become rigorous, and when IPA is stored in a holding container for a long time, the holding container in which the total amount of metal impurites dissolved in high purity IPA should be 10 pg/ml (ppt) or below is required.
Conventionally, in order to prevent the dissolving of metal impurities from the surface of stainless steel, methods of polishing the surface of the stainless steel to make the surface smooth, for example a method of performing buffing polishing or electrolytic polishing, have been disclosed. Furthermore, Japanese Laid-Open Patent Publication No. 268599/1995 discloses subjecting the surface of a stainless steel to electrolytic polishing, carrying out heat-treatment at a high temperature, and forming a colored oxide coating on a finished surface.
However, when a stainless steel is subjected to a polishing treatment (buffing polishing and electrolytic polishing), the treated product contains the amount of a metal dissolved to a level of ppm to ppb, and the attained level does not satisfy the desired target in the present invention.
Furthermore, the method comprising electrolytic polishing stainless steel, performing heat-treatment at a high temperature and forming a colored oxide coating on a finished surface gives an extremely small amount of metal dissolved, but when the dissolving of the metal is suppressed to a very low level, the electrolytic polishing treatment is required to have a high accuracy, and furthermore, the treatment takes much time and labor. Furthermore, it is difficult to apply the above method to metal containers other than stainless steel.